This invention relates to an optical data storage disc and particularly but not solely to an optical data storage disc in the form of a compact disc (CD) which carries an audio recording. The invention also relates to a method of manufacturing such optical data storage discs.
In forming recorded compact discs, firstly the digital information, primarily audio, is recorded onto a master disc by forming the surface of the master disc with pits of varying circumferential lengths, but uniform width and depth. These pits are formed by selectively exposing a surface coating on the disc with a laser beam, typically using a blue line such as the 457. 9 mm line from an Argon laser. Typically the surface coating comprises a high resolution positive photoresist, formed on a glass substrate. The areas which are exposed by the laser beam are developed using a caustic developer solution which removed the photoresist coating over those areas. The depth of the pits thus formed by the developer is normally in the range 0.1 to 0.2 microns. The exposed and developed master disc then has a thin conductive layer usually silver deposited onto the pits, normally by vacuum deposition although spray silvering and other techniques may be used. The master disc is then placed in an electroforming tank, where a layer of nickel is formed over the silver layer. The master disc is then put through a number of stages, known in the art, to produce a nickel stamper. This stamper is then applied to polycarbonate blank discs, either by pressing or vacuum forming, and the polycarbonate discs then receive a layer of aluminium and a protective coating of lacquer, which is subsequently printed upon. The process thus provides compact discs in playable form.
In forming embossed holograms, a positive photoresist coated on a glass substrate is exposed using a blue line from a laser. The process comprises recording an interference pattern between two or more wavefronts of laser light, created under extremely stable conditions and normally on an isolation table. The interference pattern, or interference fringes, provide light and dark areas, often with complex nuances which, when developed, redirect light to produce three dimensional images or other effects. The interference fringes which are recorded are often of 0.3 microns or less in depth and have a pitch normally in the range of 0.5 microns to 5 microns. In forming a holographic recording the photoresist is partially exposed over all of its area, with different regions receiving different levels of exposure involving complex phase relationships. Because of this, and also because the diffraction efficiency is reduced if the photoresist is removed right down to the substrate, generally a relatively thick photoresist coating is used, typically 1 to 2 microns thick. Also a developer is chosen which brings out the subtleties of exposure, rounding the edges of the pits to an extend which would be excessive for digital information, which requires pits with very sharp edges. The exposed and developed master is then subjected to electroforming etc. and a nickel stamper is produced, generally as for the compact disc master, and holograms can be formed by pressing or injection moulding in much the same way as for compact discs, though this is not normally the case.
Thus, traditionally the duplication of a relief hologram onto another substrate is done by embossing, using heat and pressure. The majority of current techniques for the mass production of relief holograms involve rotary techniques, in which nickel shims (thin nickel plates in the region of 35 to 100 microns in thickness) are mounted onto a roller, using either mechanical fixing or various adhesive systems, and then the embossing rollers and consequently the shims are heated (using oil or electric heaters), so that the shims are able, with the aid of pressure, to emboss various substrates. The substrates are normally embossed and subsequently metallised or they may be embossed after the material has already been metallised. Alternative methods such as casting, solvent embossing (using solvents to soften various plastics prior to embossing), and molding have also been used successfully.
It will be appreciated from the foregoing that the considerations required for producing optical data storage discs with high quality digital e.g. audio recordings, are quite different from and incompatible with the considerations required for producing holograms with high quality optical recordings.
However, we have now devised an optical storage disc which carries both a digital recording and a hologram, and we have devised a method of manufacturing such discs.